Tire thump measuring apparatus



Jan. 21, 195s D. C. AFPS 2,820,361

-A TIRE THUMP MEASURING APPARATUS Filed may 1o, 195:5v

2 Sheets-'Sheet 2 5+ ATTORNEY United States I e 2,820,361 e Y 'rms rHUMr MEASURING APPARATUS David C. Apps, Milford, Mich., assignor tov General Motors Corporation, Detroit, Mich., a corporation of Delaware e p e V AApplication May 10, 1955, Serial No. 507,314

3 Clainu. (CYL 73-67) This invention relates to an vimproved tirethump meas uring apparatus of the character described in my copending application S. N. 309,124 tiled on September ll, 1952, now Patent No. 2,735,292, of which this application is a continuation in part.

As described in the above application, tire thump is an acoustical disturbance which occurs over a portion of the operating speed range of a motor vehicle' and is most readily heard when travelling down a smooth tarvia road as,one thump for each wheeljrevolution, much as if r a bootwere present in one of the tires. This disturbance is attributed to the vehicle tires and is due to the simul'- taneous excitation of at least two audio frequency sound quency components from the other vibrations produced by a vehicle, detected the modulation envelope of the beat resulting from these components and displayed the amplitude of the modulation envelope on an indicating instrument having an arbitrary scale expressed in terms of thump number. It was reasoned that this reading would be directly related tothe personal rating that an average individual would placefupon thump.

Based upon a number of psychoacoustic tests in which the characteristics of human response associated with the hearing of beats were investigated, it wasfound that the sensitivity of the human ear varies with changes in the frequencies of the interfering components, Withchanges inthe frequency of the beat frequency and with changes in amplitude of two interfering-components of unequal pressure amplitudes. y As a result of these tests it was 1 found desirable to incorporate several unusual response characteristics inV the prede'modulation, demo'dulation, and post demodulation stages of the measuring apparatus in order to provide an accurate instantaneous correlation of the displayed values of tire thump with the personal reaction of an individual observer to thump'over a wide variety of conditions.

Accordingly, the present invention has for its general object to provide animproved tire thump measuring apparatus. More specilioally the invention seeks to provide 2,820,351. I Patented Jan. 21, 1958 2 Fig. l is a functional block diagram of a basic form of tire thump measuring apparatus;

Figs. lA to '1D illustrate wave forms of signals translated by different parts of the measuring apparatus of H response characteristic of Fig. 2.

eferring to the drawings, there is illustrated in Fig. l the principal components of a tire thump meter which includes in the relative order named, a transducer 10,V amplifier 12, first filter 14, demodulator 16, second lter 1S and an indicating instrument 20.A

The transducer lmay be a suitable vibration pickup, preferably, a sound microphone pickup of the crystal or condenser variety having a good low frequency re'- v sponsethat is substantially flat over a frequency range of, l

say, from about to 70 C. P. YS, corresponding to the observed range of wheel rotational speeds (2O to 5B1 lication of modulated waves.

miles per hour) over which the aforementioned interfering higher order vibrations, which produce tire thump, are excited. AThe amplifier 12 is an audio frequency amplit'ier that would be of the resistance-coupled variety designed in accordance with conventional methods and should have a dat low end response from, say at least 2O to 70 C. P. S. The rst filter 14 is a band-pass filter which is designed to pass'frequencies lying in a bandl which extends from about 2O to 70 C. P. S. y

, In its simplest form, the detector 16 may be a balanced linear detector that provides full wave detection-.or recti- The second filter 18 is a low-pass lte'r designed to have an upper cut-off fre# quency of around 10 C P. S. which corresponds to wheel.

a design for such measuring apparatus that takes into acbe present therein.

speeds of around 50 M. P. H. above which tire thump hasbeen found to lose its. low speed characteristic as far as the ear is concerned and partakes of other forms.-

The indicating instrument 20 may be an A. C. vacuum tube voltmeter altered to have a long time constant to eliminate meter hand flutter and to facilitate reading at the low frequencies involved.

In the operation of the above described appaartus for the measurement of tire thump, the vehicle is driven, preferably, down 'a smooth tarvia road in order to reduce other disturbances to a minimum and the microphone pickup 1D is installed in the vehicle in -a position at or near the kear of theoriver of the'vehicle, The microphone serves to senseand convertY the complex sound vibration pattern produced by the vehicle into electrical waves representative thereof indicated generally at Fig. 1A. These electrical waves will include the tire thump signal carried by the principal higherA order. harmonic vibrations generated by the tires in addition to many extraneous, vibrations produced by the vehicle. No attemptis made in the wave of Fig. 1A to accurately depict the composite form of the After amplification in the amplifier 12, the electrical waves are applied to the band-pass filter-14, which serves to acceptV and to pass therethrough all signal components lying in a band of frequencies of from about 20 to 70 C. P. S. wide and to reject the extraneous component mentioned above.. Experience has indicated that tire thump is rarely conveyed by a carrier outside of this i frequency range. The output of the band-pass filter may p many components which may correspond to the wave form shown in Fig. lB in which the familiar beat pattern is apparent.

The band of accepted frequencies is then demodulated or rectified by the full wave detector 18 to produce a wave illustrated in Fig. 1C having twice the frequency of the carrier of Fig. 1B. After filtering out the carrier tone or tones with the low-pass filter 18, the original envelope or contour is recovered and applied to the indicating instrument 20 to obtain a measure of the amplitude thereof. ln order to prevent the D. C. component of the detector output from influencing the measurements, the low-pass filter 18 could be designed to have a lower cut-off limitY of around 2 C. P. S. which is below the tire thump frequency range.

ln order that the values of thump displayed on the measuring apparatus will conform more nearly to human judgment of tire thump over a wide variety of conditions, the present invention species the use of frequency weighting considerations in the band-pass filter 14 and the lowpass filter 18 and a demodulator having a dual response characteristic. These considerations were dictated by the following psychoacoustic tests.

lt was found that when two beat component frequencies, having equal pressure amplitudes and producing a constant beat frequency of, say, C. P. S., were varied in frequency over a range from 20/25 C. P. S. to 65/70 C. P. S., the beat frequency at the lower end of this frequency range sounded louder than the beat frequency at the higher end. Thisindica'tes th'e requirement for loudness matching or equalization over the frequency range of the beat components, and the use of a rising response characteristic in the band-pass filter 14 as indicated by the dotted line of Fig. 1E in lieu of the previously provided flat-topped response characteristic shown by the solid line. The dotted response characteristic rises a't the rate of 9 db per octave and may be most readily produced cy the use of a frequency selective feedback network in the band-pass lter of Fig. 1.

Some frequency weighting of the post demodulation or low-pass filter circuit 1S appears to be dictated due to the increase in ear sensitivity as the beat frequency increases in frequency. This was observed by varying two component frequencies of equal pressure amplitude about a futed center frequency of, say, 48 C. P. S. to vary the beat frequency 'om about 3 to 10 C. P. S. This test indicated the use of a slightly decreasing or falling response characteristic for the low-pass lter in the nature of that shown in Fig. 1F.

The use of a dual response characteristic in the demodulater 16 was required by noting that the ear responds approximately linearly when there is an increase in the amplitudes of two beat components of equal pressure amplitudes, and non-linearly when the beat components are of unequal amplitudes. Thus, when two beat components of equal pressure amplitudes are, say, doubled in amplitude. the depth'of modulation or the amplitude of the modulation envelope of the resulting beat is doubled, and the 'unpression of thump severity to the human ear is doubled. This justifies the use of a substantially linear demodulation response characteristic. as represented by the curve A of Fig. 2 under these conditions.

On the other hand, when the two frequency components producing a'beat are not equal in amplitude, the impression of thump severity is not directly proportional to the depth of modulation but is a function of both theY rati'o of the two signal amplitudes and the depth of modulation. It is easy to show mathematically that with two signals of unequal amplitude the depth of modulation is always equal to the amplitude of the smaller signal. If

then, with a linear demodulator, we .start at point C in Fig. 2 and hold one component fixed and increase the n nected as a cathode-follower.

fier function to develop the desired envelope signal across load resistor RL. The triode section of tube 30 has its grid circuit `connected to the output of the band-pass filter 14 and performs a phase inversion function for the diode or detector section of the 'tube 32 serving as one section of a balanced detector. The RC networks, which are composed of resistors R1, R2, and condenser C connected between the plate and cathode of each of the diodes, perform a self-biasing function on the diode plates. They operate in such a manner that as the ratio between two signals producing a beat envelope increases, that is, if one signal is held constant and the second signal is increased in level, the self-biasing action of these networks will cause an increase in the amplitude of the envelope signal. Thus, the amplitude of the envelope signal developed across load resistor RL is a function of both the amplitude of theA envelope and the ratio of the amplitudes of the twov signals producing the envelope. The resulting voltage developed across resistor RL is applied to triode tube 34 which is connected as a cathode follower tothe input of the low-pass filter 18, whose output ter-v minals are connected to the meter 20.

ln the circuit of Fig. 4, the 10 C. P. S. low-pass filter 18 is connected to the demodulator 16 Yot' Fig. 1 and has its output fed to a phase inverter tube 40 and also to the control grid o'f` a variable-mu pentode tube 42. The output of the phase inverter feeds a full-wave rectifier shown at 44, which develops a D. C. voltage e1 across the load resistance 46. This voltage is proportional to the amplitude of the beat envelope, since only the beat envelope appears in the output of the low-pass filter 18. The input to the filter 18 is fed to tube 52 which is con- This signal contains both the desired beat envelope and the carrier frequency components represented. The output of the cathode-follower tube 52 is then fed to a full-wave rectifier bridge S4, which develops a voltage e2 across load resistance 56. The magnitude of the voltage e2 is a function of both the beat envelope and the carrier signal. The two voltages, el and e2, are connected in series-opposing and the resultant voltage is applied to the cathode of the variablemu pentode tube 42.

Circuit constants can be so proportioned that for any condition where two voltages producing a beat signal are equal, the voltages e1 and e2 will also be equal and 4a zero resultant voltage will appear at the cathode of tube 42. If, however, one signal is held constant and the otherincreased, the voltage e2 will exceed that of e1 and a positive voltage will appear at the cathode of tube 42, increasing the gain of this tube. By proper adjustment of the difference voltage (e2-e1) it is possible to obtain any desired response characteristic for the ratio network.

N it is to be understood that the above-described arrange-Y Y ments are illustrative of the application of the principles of the invention. Numerous other arrangements may be revised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

l. Apparatus for obtaining an objective measure of the subjective response lof the normal human ear to the intensity of tire thump vibrations resulting from the beating of successive higher order vibrations which are produced by the tires of a vehicle and are harmonically related'to the rotational speed of the wheels thereof, said apparatus comprising, in combination, transducer means between said higher order vibrations, second filter means connected to said detector means eiective to pass-.fre quencies in the range over which tire thump vibrations are encountered, and indicating means connected to said spouse characteristic that rises with increasing frequency in accordance with the response characteristic of the normal human ear over the pass band of said irst filter and said second filter means having a response characteristic as the modulationv envelope of the beat wave occurring 10 'ond iilter'means, said detector means having a demoduvsecond filter means, said tirst filter means having a rel5 that falls with increasing frequency in accordance with -20 the response characteristic of the normal human ear over the range of tire thump vibrations, said detector means having a demodulation response characteristic that varies approximately linearly with the amplitude of said modulation envelope when components of said lbeat wave are 25 of substantially equal amplitude to correspond to the response of the normal human ear toamplitude change of the equal amplitude components and that varies nonlinearlyfwhen said components are of diierent amplitudes to correspond to therespcmsel-ofl thenoiihal 'human' ear'130 to amplitude change of said componentsA of different amplitudes.

2. Apparatus for obtaining an objective measure of the subjective response of thenonnal human ear to thev ing of successive higher order vibrations which are produced by the tires of a vehicle and are harmonically related to the rotational speed of the wheels thereof, said apparatus comprising, in combination, transducer means 'intensity of tire thump vibrations resulting from the beat- 35 for sensing and converting the vibrations produced by 40 said vehicleintoelectric waves representative thereof, rst lter means connected to said transducer means for t 6 separating from said electrical waves components thereof lying within a frequency band corresponding to the range over which said higher order vibrations are excited and produce said tire thump vibrations, detector means connectedto said rst filter means for detecting the amplitude of said tire thump vibrations appearing as the modulation envelope of the beat wave occurring between said higher ordervibrations, second filter means connected to said detector means effective to pass frequencies in;

the range' -over which tirethulnp `vibrations are encountered, and indicating means connected to said sec lation response characteristic that varies approximately linearly with the amplitude of said modulation envelope when components of said beat wave are of substantially equal amplitude to correspond to the response of' the normal human ear to amplitude' change of the equal amplitude components and that varies non-linearly when the vbeating components are of different amplitudes to cor respondto theresponsej 'of the normal 'human ear to',

amplitude change of said components of different am pltudes.

3. Apparatus for obtaining an objective measurement of the subjective response of the normal human ear to' acoustical vibrations produced by heterodyning action of component'waves of unequal amplitude'comprising means.

forconverting' the acoustical vibrations to an electrical.k

wave having a modulation envelope corresponding to the acoustical vibrations, detector means coupled to the first i mentioned means and; including circuit meansresponsive to the amplitude ratio of said campement waves for modi.

fying the output voltage of the detector means in a non-- linear manner to correspond with the responseu of the normal human ear to amplitude change of said component nected with the detector means and responsive to -sad output voltage. t

References Cited in the file of this patent Article by H. H. Scott, Noiseinlndush'yf' in pub. l

InstrumentsV September 1937 issue, P12-23.1436.

`waves of unequal amplitude, and indicating means Acout- U. S. DEPARTMENT 0F COMMERCE PATENT OFFICE CERTIFICATE CF CORRECTION Patent No.` 2,820,361 David C., Apps lJelmer-.5r 2l, 1958 It s hereby certified that error appearsl n the printed specification of the above numbered patent requiring correction and that the said Let ners Patent should read as Corrected below.

signed and sealed this 10th day of June 1958.,

KARL HQAXLINE ROBERT C. WATSON Attesting Officer v Conlnssioner of Patents 

